The present invention relates to a multi-color image forming method employing electrophotographic copying process and its apparatus.
Conventionally, as a multi-color image forming apparatus employing electrophotographic copying process, a two-color image forming apparatus, for example, is proposed in which two sets of copying processes are provided around a photosensitive member and each of the copying processes includes a corona charger for charging a surface of the photosensitive member to a predetermined potential, an exposure device for irradiating image light onto the photosensitive member charged by the corona charger and a magnetic brush type developing device utilizing two-component developer composed of toner and carrier. Thus, in the first copying process, a first electrostatic latent image formed on the photosensitive member is developed by first toner so as to form a first toner image. Meanwhile, in the second copying process, a second electrostatic latent image is developed by second toner having not only a color different from that of the first toner but charging characteristics relative to the carrier, identical with those of the first toner so as to form a second toner image such that the first and second toner images on the photosensitive member are transferred at a time.
However, in the known two-color image forming apparatus, such a problem arises that since the second developing device is of magnetic brush type in which development is performed through contact of a magnetic brush with the surface of the photosensitive member, the first toner image formed in the first copying process is brought into contact with the second toner having the color different from that of the first toner, in the second copying process and thus, the second toner is mixed into the first toner image, thereby resulting in mixing of the colors in the first toner image.
Therefore, in order to prevent mixing of the second toner into the first toner image in the known two-color image forming apparatus, such measures have been taken that a developing bias applied to the second developing device is made higher than that of the first developing device as proposed in U.S. Pat. No. 4,416,533 or in the second copying process, a surface potential of the photosensitive member and a surface potential of the first toner image are made higher than the developing bias of the second developing device.
However, even if the above mentioned measures are taken, it is still impossible to eliminate mixing of the second toner into the first toner image. Thus, when a quantity of mixing of the second toner into the first toner image exceeds a certain value, presence of the second toner in the first toner image becomes conspicuous, thereby resulting in deterioration of image quality.